Solar cells and modules are photovoltaic (PV) devices that convert sunlight energy into electrical energy. The most common solar cell material is silicon (Si). However, lower cost PV cells may be fabricated using thin film growth techniques that can deposit solar-cell-quality polycrystalline compound absorber materials on large area substrates using low-cost methods.
Group IIB-VIA compound semiconductors comprising some of the Group IIB (Zn, Cd, Hg) and Group VIA (O, S, Se, Te, Po) materials of the periodic table are excellent absorber materials for thin film solar cell structures. Especially CdTe has proved to be a material that can be used in manufacturing high efficiency solar panels at a manufacturing cost of below $1/W.
FIG. 1 shows a commonly used structure of a CdTe based thin film solar cell. FIG. 1 shows a “super-strate” structure 10, wherein light enters the active layers of the device through a transparent sheet 11. The transparent sheet 11 serves as the support on which the active layers are deposited. In fabricating the “super-strate” structure 10, a transparent conductive layer (TCL) 12 is first deposited on the transparent sheet 11. Then a junction partner layer 13 is deposited over the TCL 12. A CdTe absorber film 14, which is a p-type semiconductor film, is next formed on the junction partner layer 13. Then an ohmic contact layer 15 is deposited on the CdTe absorber film 14, completing the solar cell. As shown by arrows 18 in FIG. 1, light enters this device through the transparent sheet 11. In the “super-strate” structure 10 of FIG. 1, the transparent sheet 11 may be glass or a material (e.g., a high temperature polymer such as polyimide) that has high optical transmission (such as higher than 80%) in the visible spectra of the sun light. The TCL 12 is usually a transparent conductive oxide (TCO) layer comprising any one of; tin-oxide, cadmium-tin-oxide, indium-tin-oxide, and zinc-oxide which are doped to increase their conductivity. Multi layers of these TCO materials as well as their alloys or mixtures may also be utilized in the TCL 12. The junction partner layer 13 is typically a CdS layer, but may alternately be compound layer such as a layer of CdZnS, ZnS, ZnSe, ZnSSe, CdZnSe, etc. The ohmic contact 15 is made of a highly conductive metal such as Mo, Ni, Cr, Ti, Al or a doped transparent conductive oxide such as the TCOs mentioned above. The rectifying junction, which is the heart of this device, is located near an interface 19 between the CdTe absorber film 14 and the junction partner layer 13.
Ohmic contacts to p-type CdTe are difficult to make because of the high electron affinity of the material. Various different approaches have been reported on the topic of making ohmic contacts to CdTe films. For example, U.S. Pat. No. 4,456,630 by B. Basol describes a method of forming ohmic contacts on a CdTe film comprising etching the film surface with an acidic solution, then etching with a strong basic solution and finally depositing a conductive metal. In U.S. Pat. No. 4,666,569 granted to B. Basol a multi layer ohmic contact is described where a 0.5-5 nm thick interlayer of copper is formed on the etched CdTe surface before a metallic contact is deposited. U.S. Pat. No. 4,735,662 also describes a contact using 1-5 nm thick copper, an isolation layer such as carbon, and an electrically conducting layer such as aluminum. U.S. Pat. No. 5,909,632 describes a method of improving contact to CdTe by depositing a first undoped layer of ZnTe, then depositing a doped ZnTe layer, such as metallic Cu as the dopant at concentrations of about 6 atomic percent, and finally depositing a metal layer. U.S. Pat. No. 5,472,910 forms an ohmic contact by depositing a viscous liquid layer containing a Group IB metal salt on the CdTe surface, heating the layer, removing the dried layer and depositing a contact on the surface. U.S. Pat. No. 5,557,146 describes a CdTe device structure with an ohmic contact comprising a graphite paste containing mercury telluride and copper telluride.
FIG. 5 schematically shows an interface 52 formed between a CdTe layer 50 and a ZnTe layer 51 used for the formation of an ohmic contact. The prior art ZnTe layer 51 in this example is grown on top of the CdTe layer 50 by a prior art method such as sputtering or evaporation from a ZnTe source. The CdTe grains 53 of the CdTe layer 50 may be 1-5 microns in size. The ZnTe grains 54 of the ZnTe layer 51 may have a size of about 0.1-1 microns. Since the ZnTe layer 51 is grown on top of the already grown CdTe layer 50, the interface 52 between the two layers is very sharp and defective and it comprises electronic surface defects of the CdTe layer 50. In other words, there is a discontinuity and a break in the grain structure going from the CdTe layer 50 into the ZnTe layer 51. This break in the polycrystalline structure and lack of epitaxy introduces many electrically active interface states that act as recombination centers with high hole-electron recombination velocities, such as recombination velocities of 106 cm/sec or higher. A CdTe solar cell with an absorber thickness of less than about 1.5 microns would not yield high efficiency if it is constructed with a defective CdTe/electron reflector interface such as the one shown in FIG. 5.
The present inventions provide improved ohmic contacts to CdTe films and facilitate the fabrication of ultra-thin devices.